The present invention relates generally to structures having a refractive index distribution and in particular to an optical medium having an arbitrary desired effective refractive index distribution using at least two materials with thicknesses less than a wavelength.
Optical communication systems require optical components to guide and/or manipulate a light beam in continuous or pulse format. Of the various optical components used in optical communication systems, some have a continuous change in the distribution of the refractive index. Examples include graded refractive index multimode optical fibers [e.g., Cohen et al., “Multimode Optical Fiber,” U.S. Pat. No. 3,989,350, Nov. 2, 1976; Fleming, Jr., “Multimode Optical Fiber,” U.S. Pat. No. 4,033,667, Jul. 5, 1977 and graded refractive index rod lenses [e.g., Ho Shang Lee, “Miniaturization of Gradient Index Lens Used in Optical Components,” U.S. Pat. No. 6,088,166, Jul. 11, 2000]. A continuous refractive index change can serve various purposes, including light beam collimation, focusing, imaging and the like [Duncan T. Moore, “Gradient Index Optics: A Review,” Applied Optics, Vol. 19, No. 7, 1 Apr. 1980].
Traditionally, such graded refractive index (“GRIN”) devices have been created by mixing different materials. Most of these methods involve a dominant material and another material that is either used as a dopant or dispersed in the dominant material [e.g., Park et al., “Production Method for Objects with Radially-Varying Properties,” U.S. Pat. No. 6,267,915 B1, Jul. 31, 2001]. As there is generally a limit to the amount of dopant that can be uniformly distributed inside the dominant material, the maximum refractive index change (Δn) that can be achieved using such a method is generally less than 0.1. As a result, when such a method is used to make a graded refractive index lens to focus a light beam, the light focusing power of the lens is generally low. In other words, the focal length of such a lens is relatively long, and the focused beam spot size is relatively large (typically a few microns). A typical example is an ion exchanged glass based GRIN rod lens, such as lenses made by GRINTECH GmbH, for which the focal length (also called a quarter pitch) is on the order of millimeters and the focused beam spot size is on the order of a few microns.
Various techniques for producing GRIN devices having an essentially continuous refractive index change have been developed. A first method uses neutron irradiation, in which a boron rich glass is bombarded with neutrons to create a change in the concentration of boron and hence a change in the refractive index of the material [P. Sinai, Applied Optics, Vol. 10, pp 99 (1971)]. This method has limited application because the gradient is not permanent, and the maximum index change is only about 0.02.
A second method uses chemical vapor deposition (CVD) to create a fiber preform for a GRIN optical fiber. A glass material of a given refractive index is deposited on either the inside or outside of a tube. A succession of layers having slightly different chemical compositions (e.g., slightly increased or decreased dopant concentrations) is then deposited. Each layer generates a small step in the refractive index. After the preform, which typically has a diameter of about 2.5 cm, is made, a fiber is drawn from it. As the fiber is drawn, the layers become so thin that the refractive index distribution becomes effectively continuous [e.g., Cohen et al., “Multimode Optical Fiber,” U.S. Pat. No. 3,989,350, Nov. 2, 1976; Fleeting, Jr., “Multimode Optical Fiber,” U.S. Pat. No. 4,033,667, Jul. 5, 1977; Dabby et al., “Graded Start Rods for the Production of Optical Waveguides,” U.S. Pat. No. 4,298,366, Nov. 3, 1981; Dabby et al., “Graded Optical Waveguides,” U.S. Pat. No. 4,423,925, Jan. 3, 1984]. The maximum index change that can be achieved using this technique is about 0.01.
A third method involves producing a graded refractive index in organic or plastic materials. For instance, multiple plastic layers of successively increasing or decreasing refractive index can be deposited on a plastic cylinder or planar surface; the material is then cured [e.g., Toyoda et al., “Distributed Graded Index Type Optical Transmission Plastic Article and Method of Manufacturing Same, “U.S. Pat. No. 5,390,274, Feb. 14, 1995; Nakamura, “Method and Apparatus for Manufacturing Distributed Refractive Index Plastic Optical Fiber,” U.S. Pat. No. 6,132,650, Oct. 17, 2000]. In related processes, monomers can be changed to polymers or cross linking of polymers can be enabled using either thermal radical polymerization (induced by UV or laser light, photodimerization, or electron ray) or condensation polymerization (induced by radical addition or photodimerization) [e.g., Jung et al., “Manufacturing Method of a Polymer GRIN Lens Using Sulfonation,” U.S. Pat. No. 5,567,363, Oct. 22, 1996]. The disadvantages associated with plastics include the relatively strong absorption of light in the standard communication wavelength band, as well as the relatively low thermal and lifetime stability of the plastic material. Although attempts to address these problems—e.g., by replacing the C—H bond by a C—F bond [Sugiyama et al., “Graded Refractive Index Optical Plastic Material and Method for its Production,” U.S. Pat. No. 6,166,125, Dec. 26, 2000]—have been somewhat successful, the optical communication industry still generally prefers not to use plastic gradient lenses.
A fourth method, ion exchange, is commonly used to make glass-based graded refractive index rod lenses. Ions from a molten salt, e.g., lithium bromide or potassium nitrate, diffuse into glass and are exchanged with larger ions in the glass [e.g., Senapati et al., “Graded Index Lens for Fiber Optic Applications and Technique of Fabrication,” U.S. Pat. No. 6,128,926, Oct. 10, 2000; Senapati et al., “Graded Index Lens for Fiber Optic Applications and Technique of Fabrication,” U.S. Pat. No. 6,172,817 B1, Jan. 9, 2001]. The possible refractive index distribution is limited by the diffusion process, and the maximum refractive index change is only about 0.05.
A fifth method is ion or molecular stuffing, in which heat is applied to separate the phases of a special glass in which one of the phases is dissolvable in an acid. After acid treatment, the glass is immersed in a bath to allow other ions or molecules to diffuse into the glass pores; alternatively, the pores may be left behind. The glass is then condensed by heating [e.g., R. K. Mohr et al., Digest of Topical Meeting on Gradient-Index Optical Imaging Systems (Optical Society of America, Washington, D.C., 1979), paper WA I; Macedo et al., “Method of Producing Optical Wave Guide Fibers,” U.S. Pat. No. 3,938,974, Feb. 17, 1976. However, most glasses that are well accepted for optical communication do not possess the required uniform phase separation property. In addition, the maximum refractive index change is still only about 0.05.
A sixth method involves a “sol-gel” technique, in which a solution of multiple metal oxides is spin coated or dip coated on a surface. Heat treatment follows to evaporate the solvent and condense the metal oxides, thereby forming a thin layer of glass film. To create a graded refractive index multiple layers having different metal oxide content can be successively deposited. One problem with this technique is that cracking of the film tends to occur as the film grows thicker and the amount of dopant in the solution increases. It is thus difficult to produce a film thickness of the order of about 10 μm. In an alternative process, a gel glass film or gel glass rod can be made first, and glass phase separation can be induced by heat treatment followed by selective leaching or etching to create a microporous structure with a graded refractive index [e.g., McCollister et al., “Process of Making Glass Articles Having Antireflective Coatings and Product,” U.S. Pat. No. 4,273,826, Jun. 16, 1981; S. P. Mukherjee et al., “Gradient index AR Film Deposited by the Sol Gel Process,” Applied Optics, Vol. 21, No. 2, p. 283, Jan. 15, 1982; S. Konishi et al., “r-GRIN Glass Rods Prepared By a Sol Gel Method,” Journal of Non Crystalline Solids, Vol. 100, pp. 511 513 (1988); Debsikdar, “Broadband Antireflective Coating Composition and Method,” U.S. Pat. No. 4,839,879, May 16, 1989]. Again these methods can only produce a maximum index change of about 0.1.
A seventh method involves UV imprinting of photosensitive glasses to disrupt the metal oxygen bond, thereby inducing a refractive index change. This technique is commonly used to make fiber Bragg gratings (FBG), which are common components in today's fiber-optic communication systems. The most commonly used material is germanium doped silica fiber. After hydrogen loading and UV laser imprinting, the maximum index change is about 0.1. Another glass of higher photosensitivity is lead oxide, but it has been found that the maximum achievable index change for UV imprinting of lead oxide is only about 0.2 [see the above-referenced co-pending U.S. patent application Ser. No. 09/884,691.
An eighth method makes use of centrifugal force applied during the combustion synthesis of composite materials, with the result that the composition and particle size of the metallic or ceramic component changes continuously across the thickness of the product [e.g., Munir et al., “Centrifugal Synthesis and Processing of Functionally Graded Materials,” U.S. Pat. No. 6,136,452, Oct. 24, 2003. This method involves applying a glass melting high temperature to a rotating mold containing glass powder, mixtures. As in the glass melting methods involving dopants, there is a maximum concentration of the metallic or ceramic component that can be achieved without clustering. Consequently, the maximum index change that can be achieved is small (about 0.1).
None of these methods is able to produce a maximum index change greater than about 0.2. In addition, none of these methods is able to produce an arbitrary refractive index profile with high precision because these methods generally rely in some way on diffusion of one material into another, which cannot be precisely controlled. In some applications, for instance, light coupling between a single mode optical fiber and a III-V compound semiconductor waveguide, a graded refractive index device with a maximum refractive index change greater than about 0.2 would be desirable. Such a device would provide higher focusing power and a smaller focused spot size than GRIN devices produced by existing techniques.